1. Field of the Invention
The present invention relates to a spectrophotometric method of irradiating a sample solution with light to measure a transmitted light therefrom, and a spectrophotometric apparatus for use in the spectrophotometric method. More specifically, the present invention relates to a spectrophotometric method and apparatus suitable for spectrophotometrically measuring a small volume of sample solution.
2. Description of the Background Art
In a process for measuring a transmittance or absorbance of a sample solution using a spectrophotometer, such as an ultraviolet-visible spectrophotometer, a rectangular or cylindrical cuvette cell is generally used for containing the sample solution. A typical cuvette cell has an inner volume of several mL or more, and it is required to prepare a sample solution in a sufficient volume to fill the cuvette cell.
Recent years, an ultraviolet-visible spectrophotometer has been increasingly used in the field of biochemistry. In this field, it is often the case that a sample solution for analysis is very small in volume. Particularly, in analyses of DNA, some analyses have to be conducted using only several μL or less of sample solution, due to scarcity and costliness of a sample. Heretofore, there has been known a vessel suitable for spectrophotometrically measuring such a small volume of sample solution [see, for example, Japanese Patent Laid-Open Publication No. 05-302893 [Patent Publication 1)], because the above cuvette cell cannot be used for the purpose of analyzing a small volume of sample solution.
As a cell for use in measuring a small volume of sample solution, a capillary cell has been commonly known that is designed to suck up and hold a sample solution based on a capillary phenomenon. Generally, the capillary cell still requires preparing several μL or more of sample solution at minimum, and cannot be used for any analysis of a sample solution with a volume of less than the lower limit. The capillary cell has another problem about the needs for filling the cell with a sample solution in a complicated manner and taking a lot of time and effort in cleaning after measurement.
As a device allowing for spectrophotometry of a sample solution with an extremely small volume of about 1 μL, there has been known a spectrophotometer ND-100 available from NanoDrop Technologies Inc., USA [see “NanoDrop ND-1000 Overview”, NanoDrop Technologies Inc., URL: http://www.nanodrop.com/nd-1000-overview.html (Non-Patent Publication 1)]. As shown in FIG. 9, this spectrophotometer has an upper pedestal 50 and an lower pedestal 52 disposed in vertically opposed relation to one another with a given distance therebetween, and a sample solution 54 is held in a space between the upper and lower pedestals 50, 52 based on a surface tension in a vertically bridging manner. Then, a measurement light is emitted from a light-emitting optical fiber 51 incorporated in the upper pedestal 50 to pass through the sample solution 54, and a transmitted light is received by a light-receiving optical fiber 53 incorporated in the lower pedestal 53. A light path length in the sample solution is set at about 1 mm. It is described that this spectrophotometer is usable in analyses of a sample solution with an extremely small volume of about 1 to 2 μL.
This spectrophotometer involves a time-consuming operation for cleaning respective end faces of the light-emitting and light-receiving optical fibers after completion of a measurement of one sample to perform a measurement of another sample (the Non-Patent Publication 1 describes as “wipe using a laboratory wipe”). While it is desirable to clean the end faces using water or organic solvent because the wiping operation using a laboratory wipe is not sufficient to completely eliminate an influence of a previously-measured sample solution, such a cleaning operation is fairly bothersome. Moreover, this operation has to be performed for each sample measurement, or it is impossible to measure a number of samples while automatically replacing one with another. This analytic operation cannot be the to be efficient. Furthermore, the device designed to partly have a direct contact with a sample solution is likely to cause damages and contamination in the contact portions, and, in that event, an analytic performance will be deteriorated. As measures for avoiding such problems, a lot of times and efforts have to be spent for frequent maintenances and inspections of the device.